Function par_map_pages 

pub fn par_map_pages<F>(pages: &mut Vec<Vec<ContentElement>>, op: F)
where
    F: Fn(Vec<ContentElement>) -> Vec<ContentElement> + Sync + Send,

Apply op to each page in parallel, replacing each page’s content.

This is the parallel equivalent of:

for page in &mut pages {
    let elements = std::mem::take(page);
    *page = op(elements);
}